Conventionally, a tube of a single structure is normally used as a tube through which fluid such as gas flows to a turbine from a gas source. There is a tube of a double tube structure in which a tube is disposed inside another tube, the inside tube (hereinafter, referred to as “inner tube”) is inserted into the outside tube (hereinafter, referred to as “outer tube”) in a state where a constant gap is held between the inner and outer tubes, two kinds of gases sent from a supply source located upstream flow through the outer tube and the inner tube, and the gases are supplied to a predetermined discharging destination located downstream (the Publication of Japanese Patent No. 3971683).
Further, such a tube of the double tube structure is also used in a piping system in spent nuclear fuel disposal equipment, radioactive-waste disposal equipment, and nuclear power plant equipment (Japanese Unexamined Patent Application, Publication No. H09-54189 and Japanese Unexamined Patent Application, Publication No. 2003-294883).
However, the conventional double tube structure has a problem that, when the inner tube is to be disassembled in a case of maintenance of a tube connecting device or in a case of repairing operation of a damaged part of the inner tube, the inner tube cannot be disassembled unless the outer tube is cut.